introduction to sensory pathways...auditory cortex • auditory cortex consists of multiple areas...
TRANSCRIPT
Introduction to sensory pathwaysGatsby / SWC induction week
25 September 2017
Studying sensory systems: inputs and needs
Brain (Arduino)
Inputs
Outputs
Stimulus Modality Robots’ Sensors
Biological Sensors
Light Vision Photodiodes or CCD
Photoreceptors in the retina
Sound Audition Microphone Hair cells in the cochlea
Pressure Somatosensation Pressure sensor Cutaneous mechanoreceptor
Chemical Smell and taste Chemical sensor Chemoreceptor
Gravity Vestibular Accelerometer Hair cells in the vestibular labyrinth
Physical stimulus
Neuronal activity
Conscious perception
Psychophysics
Sensory physiology
Overview: dealing with different types of inputVision Audition Olfaction
Vision
Overview: dealing with different types of input
Introduction to the visual system
• Vision is one of the best studied systems in the brain
• The most important modality for us, and the most complex circuitry
• NOT a camera: the visual system solves the inverse problem
Image formation
Vision (brain)
The retina: just a CCD?
Receptive fields
Early visual processing: thalamic pathway
Primary visual cortex (V1) decomposes image into low-level features: bars with a specific orientation
Functional organisation of V1
Ocular dominance Orientation columns Color blobs
Effects of lesions in different locations
Beyond V1: dorsal and ventral streams
• Ventral “what” pathway
• Specialises in object recognition
• Includes areas V1, V2, V4 and inferior temporal areas
• Dorsal “where” pathway
• Specialises in object localisation
• Includes V1, V2, V3, MT (V5), MST and inferior parietal cortex
• Each functional area contains a full retinotopic map
Dorsal vs ventral: exampleMotion perception in V5 (MT) Perception of shapes in IT
Size invariancePosition invariance
Take-home messages from the visual system
• Retinotopic mapping: geometric placement of neurons follows organisation in the retina
• The processing hierarchy: lower order areas encode lower order features of the image. Higher order areas encode higher order features
• Receptive fields grow when you ascend the hierarchy
• The dorsal stream tells you “where”, the ventral stream tells you “what”
Audition
Overview: dealing with different types of data
Introduction to the auditory system
hammer, anvil and stirrup
16,000 hair cells
Hair cells: mechanoreceptors in the ear
Tonotopy in cochlear hair cells
Tonotopic organisation
Tuning curves for cochlear hair cells
The auditory pathways: structure and function
• Auditory information is much more transient than vision
• Picking up on small temporal differences is important
• Extensive subcortical structures implement much of this quickly before information has reached cortex
• Not one, but three pathways from cochlea to cortex
Sound localisation in the superior olive
IID: Interaural Intensity Difference ITD: Interaural Time Difference
Sound localisation based on intensity difference
Sound localisation based on time difference
Auditory cortex
• Auditory cortex consists of multiple areas and maintains a tonotopic mapping
• There are cells that respond to either ear (EE) and cells that respond to one ear, inhibited by the other (EI)
• Its function is more ambiguous than visual cortex. Does it predict future events?
Thalamus Auditory cortex Hippocampus
Rummell et al. 2016, JNeurosci
Take home message for audition
• The auditory system retains tonotopic mapping
• A lot of of auditory processing happens subcortically
• Sound localisation in the superior olive: structure serves computation
Overview: dealing with different types of inputOlfaction
Introduction to olfaction
• The olfactory system processes information about chemicals in the environment: a specific mix of chemicals constitutes an odour
• Molecules bind to Olfactory Sensory Neurons in the nose
• Given the complex mixture of chemicals, which odours are present?
Diversity in olfactory receptors
• Different olfactory sensory neurons have different kinds of receptors with different sensitivities to certain odours
• Genes in humans and rodents code for 1000 different types of odourant receptors
• Each neuron expresses only one kind of receptor
The olfactory pathways
Take home messages olfaction
• There are 1000+ receptor types in the nose
• Glomeruli in the olfactory bulb get input from a specific type of olfactory sensory neuron
• The relative location of these different glomeruli is highly genetically preserved across species
Overview: dealing with different types of inputsVision Audition Olfaction
• Goal: reconstruct 3D world from 2D image
• Most complex system: mostly cortical processing
• Functional specialisation in what and where
• Retinotopic mapping
• Goals: identify and localise sounds, speech comprehension, etc.
• Timing is crucial: extensive subcortical processing
• Tonotopic mapping
• Goal: demix odours to identify source
• Direct connections from olfactory bulb to many areas
• Preserved spatial organisation of odours